EE155/255 Green Electronics - Stanford University · EE155/255 Green Electronics Power Circuits...

EE155/255 Green Electronics

Power CircuitsPhotovoltaics

10/5/16

Prof. William DallyComputer Systems Laboratory

Stanford University

Course Logistics• HW2 due Monday 10/10• Lab1 signed off this week• Lab2 out

EE155/255 Lecture 4 - Power Circuits

Course to Date• We need sustainable energy systems• At the core they are voltage converters• Periodic steady-state analysis, buck and boost• Intelligent control + power path• Intelligent control done with event-driven embedded software• Real devices have switching and conduction loss

Last Time• DC and AC characteristics of MOSFETs, Diodes, and IGBTs• Switches in pairs• One switch does the work• Turn on transient• Diode reverse recovery• Parasitics• Gate drive and Miller capacitance• “Dead time” and “shoot through”

Review - Turn-On Loss

ILQRR QD

t1 t2 t3

Review - Effect of Miller Cap on Rise Time

=iGCDG

Δt = ΔVDCDG

Example: iG = 0.5A, C = 100pF, DV = 400V

Snubbers

Dampen Ringing Nodes

LD and Cj resonate when M is on

Parallel RS dampens tank

Series CS limits dissipation

Inductance on Drain

8uJ turn-on

42uJ turn-off

With Snubber (1nF, 5W)

8uJ turn-on

2uJ in snubber

42uJ turn-off

Design Procedure

Pick RS ~ 1/wCj

Pick CS so t >= p/w

OrEs = CSV2/2

G 50V+-

Move Turn-Off Dissipation to Passive Device

CS slows rise time of drain

CSV2/2RS dissipated in RS when CS discharges

Rarely used today

Other forms slow fall time and rising/falling currentEE155/255 Lecture 4 - Power Circuits

Critical Loop

Lab Half-Bridge Module

The Half-Bridge Module

IRS21834

ComVss

��

C14.7 F

2.2 F200V

D356V5W

VBCSupply

VDCFilter

Bootstrap Supply

IRS21834

ComVss

��

C14.7 F

2.2 F200V

D356V5W

VBCSupply

VDCFilter

Bootstrap Supply

Drain Voltage Filter

IRS21834

ComVss

��

C14.7 F

2.2 F200V

D356V5W

VBCSupply

VDCFilter

Drain Voltage Filter300nH Input Inductance

SPICE Example – A Voltage Doubler

A Voltage Doubler* Simple voltage "doubler".include "gel.lib".param td=100n tr=100n tf=100n tw=2.5u tcy=5u ncy=2.param l1=22uH c1=10uF r1=10

* call half-bridge subcircuitxhb vd mid g g 0 v12 gel_hb

* circuitl1 vin mid {l1}c1 vd 0 {c1}r1 vd 0 {r1}

* suppliesv12 v12 0 12vin vin 0 24

* stimulusVG g 0 PULSE(0 5 {td} {tr} {tf} {tw} {tcy} {ncy})

.ic i(l1)=9.2

.ic v(vd)=42.8

.tran {ncy*tcy}

Turn-On Transient

Steady State

Close up of Drain Current

With PID Control

A Warning• SPICE (or any simulator) is a Verification tool, not a Design tool• Design your circuit first

– Use Excel, Matlab, a calculator etc… to calculate component values• Then simulate your circuit to check operation and fine-tune parameters• Don’t try to design your circuit using SPICE

• Simulation is not a substitute for thinking

Summary of Power Circuits• Real switches have limitations

– Conduction losses (RON for FETs, VCE for IGBTs, Diode drop)– Switching losses (finite ton, toff, trr)

• With current source load, current ramps, then voltage falls • And voltage rises before current falls• May be dominated by reverse recovery time• Complicated by inductance

– Parasitic L and C• Power MOSFETs

– Switch quickly, have linear I-V, integral diode• IGBTs

– Diode-like I-V, slower switching• Diodes

– Have reverse recovery time• Switches operate in pairs

– For one-way converters, one switch may be a diode– Synchronous rectification – make both switches FETs to reduce loss– Need “dead time” to avoid “shoot through” current

• Gate-drive circuits control rise and fall times– Supply Miller capacitance

• Bootstrap supply needed for high-side driver• Snubbers dampen voltage and current transients• Use SPICE as a verification tool, not a design tool

Photovoltaics

Energy Conversion

Photovoltaic System

Solar Panel

Photovoltaic Array

PV Controller and Inverter

Batteries

400V DC 240V AC60 Hz

48V DC

To Grid

215 Installation and Operation

opyright � 2012 Enphase Energy

141-00012 Rev 04

ple Wiring D

iagram – M

215, 240 VA

Electrons absorb energy from photons

Equivalent Circuit

RSHISC

IV-Curve

Typical Module CS6P60 cells in series

~0.5V per cell

3 strings of 20 with bypass diode on each string

Typical Module

… … …EE155/255 Lecture 4 - Power Circuits

IV Curve from SPICE Model

Peak-Power Tracking• Find point on IV curve where power is maximized.

Start at any point (v(0),i(0))“Dither” v, v(i+1) = v(i) + DvCheck result: if(p(i+1) < p(i)) v(i+1) = v(i)Try both directions: Dv = -Dv

MPP Tracking – The Movie

Start at (35 V, 5.5A) P=192.5

Dither by DV = 0.5V to V = 35.5V(35.5V, 4.7A) P=166.9 < 192.5

(35.5V, 4.7A) P=166.9 < 192.5Bad Move – Go Back to (35, 5.5)

Dither by -0.5V to 34.5V(34.5, 6.2) P=213.9 > 192.5

(34.5, 6.2) P=213.9 > 192.5Keep move and keep going

Move to 34.0(34.0, 6.7) P=227.8 > 213.9

(33.5, 7.0) P=234.5> 227.8 Keep move and keep going

(32.5, 7.5) P=243.75 > 240.9 Keep move and keep going

(32.0, 7.6) P=243.2 < 243.75 Abandon Move and Go Back!

Operate at (32.5, 7.5) P=243.8With occasional forays to 32.0 and 33.0

“Hillclimbing” On the Power Curve

Compound Power Curve

Compound Power Curve (2 Panels)

Not convexHow do you find maximum power point?

Three Panels

Typical String of 10 PV Panels

Search Strategies for Non-Convex MPPT• Exhaustion

– Try every operating point• Random

– Randomly pick new points – keep if better• Hierarchical

– Try every point – with coarse spacing– Try every point near best point with finer spacing– Repeat

• Acquire and Track– One of the above to acquire MPPT (e.g., hierarchical)– Then gradient search to track– Periodically revisit (devote some fraction of string time to this)

• Optimal method depends on – Shape of curve– How fast the curve changes– How the curve changes

Good Optimization Depends on Understanding The Problem

• Collect lots of data– Time series of IV curves from typical strings

• Understand the data• What causes “dips”

– Bad panels • Static offset in current

– Fixed shading – trees, buildings, etc… • Periodic offset – same time each day

– Variable shading – clouds, etc… • Unpredictable shading – but shifts across panels in one direction

• Develop algorithms• Test on data

An Example of Optimization• Trade-off parameters against one another to maximize a figure of merit.

• In this case, parameters are panel voltage and current.

• Figure of merit is power.

• Optimization is done real-time because temperature and irradiance change.– Sometimes optimization is done at design time, or calibration time.

MPPT Power Path(Boost Converter with Energy Meter)

PV Panel

MPPT Power Path(Boost Converter with Energy Meter)

PV Panel

MPPT is a boost converter that regulates its INPUT voltage

Cycle Waveforms

350 355 360 365 370 375 3802

350 355 360 365 370 375 38034.5

v in (V

350 355 360 365 370 375 38043

v out (V

t (µs)

Size input cap Ci for acceptable ripple

Size output cap Co for acceptable ripple

Size inductor L to set ripple

Longer Simulation

0 2 4 6 8 10 12 14 1610

v in(V

0 2 4 6 8 10 12 14 160

i pv(A

0 2 4 6 8 10 12 14 1620

v out(V

0 2 4 6 8 10 12 14 160

0 2 4 6 8 10 12 14 1650

100150200250

t (ms)

PV Systems

Microinverter

Panel InverterAC Line240 Vrms~1Arms

30-40V0-10A

Store Energy During AC Null

Approach 1 – DC Link

30-40V0-10A

340-600V0-1A

Buck Unfold

Rectified AC240V, 1A rms

Approach 2 – Single Stage

30-40V0-10A

Convert Unfold

Rectified AC240V, 1A rms

400VDC Unfold

400-600V120Hz Buck

240V 120Hz rectified sine240V AC 60Hz

2/3 of power through main pathLower path levels input current

Two-Path

3-Phase

String ofPanels

Inverter

AC Line480 V20 A3 phase

600-1000V10A

No need for energy storage

48V34AH

3-F Inverter Power Path

Transformerless

Typical Utility-Scale PV System

Typical Utility-Scale PV System• 8,000 Modules – 400 strings of 20 modules each

– 325W/module – 2.6MW DC total• Central 2MW inverter• Central 2MW step-up transformer to 34.5kv• Single axis tracking• This 2MW “block” is repeated for larger systems

PV Economics 1• Utility scale costs

– PV Module $0.60/W– Inverter $0.10/W– Mounting $0.15/W– Balance $0.65/W– TOTAL $1.50/W

• Return– Hours/year 2,200– Wholesale $0.05/kWh– TOTAL $0.11/Wyear– 7.3% ROI

• Residential costs– PV Module $0.60– Microinverter $0.50– Mounting $0.20– Balance $1.70– TOTAL $3.00

• Return– Hours/year 2,200– Retail $0.15-$0.35/kWh– TOTAL $0.33-0.77/Wyear– 11% - 26% ROI

PV Economics 2• Module is only 40% of cost (20% for residential)• Real issue is balance-of-system (installation labor)

VOC Limiting• Typical module (Trina TSM-310-PD14)

– Vmp = 36V, Voc = 46V (worst-case cold temperature)• Inverter input limited to 1kV

– Limits strings to 21 modules– At Vmp could have 27 modules – 29% increase– Reduces string cost by ~30%.

Module (and Cell) Mismatch• String current limited to current from weakest cell• Module current mismatch s = 5%• Worse for residential installations (partial shading)

• Two questions:– What is the typical mismatch profile of a 10-module string?– What power reduction does a X % current mismatch result in?

Faults and Failures• Cell open/short• Diode open/short• Arc fault

Summary of PV• PV cells/strings are voltage-dependent current sources (Diode in parallel

with current source)• PV controllers regulate their input voltage/current to maximize power

– Maximum power-point tracking• Can apply almost any converter topology

– Boost used for illustration– Regulate input rather than output

• Gradient search for convex optimization• More sophisticated search needed for multi cell/panel string

In Upcoming LecturesNo Date Topic HWout HWin Labout Labck Lab HW

1 9/26/16Intro(basicconverters) 1 1 IntrotoST32F3 PeriodicSteadyState2 9/28/16EmbeddedProg/PowerElect.3 10/3/16PowerElectronics- 1(switches) 2 1 2 1 ACEnergyMeter PowerDevices4 10/5/16PowerElectronics- 2(circuits)5 10/10/16Photovoltaics 3 2 3 2 PVMPPT PVSPICE6 10/12/16FeedbackControl7 10/17/16ElectricMotors 4 3 4 3 MotorcontrolMatlab Feedback8 10/19/16IsolatedConverters9 10/24/16SolarDay 5/PP 4 5 4 Motorcontrol- Lab/ IsolatedConverters

10 10/26/16Magnetics11 10/31/16SoftSwitching 6 5/PP 6 5 PS MagneticsandInverters12 11/2/16ProjectDiscussions13 11/7/16Inverters,Grid,PF,andBatteries 6 P 6 Project14 11/9/16Thermal&EMI15 11/14/16QuizReview C116 11/16/16Grounding,andDebuggingQ 11/16/16Quiz- intheevening C2

11/21/16ThanksgivingBreak11/23/16ThanksgivingBreak

17 11/28/1618 11/30/16 C319 12/5/1620 12/7/16Wrapup

TBD Projectpresentations PTBD Projectwebpagedue

EE155/255 Green Electronics - Stanford University · EE155/255 Green Electronics Power Circuits...

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